For production of a halomethyl ester of an aliphatic carboxylic acid, there have hitherto been known processes by so-called chloromethylation (Blanc-Quelet reaction), such as a process described in J. Am. Chem. Soc., 89, 21, 5439 (1967) wherein pivaloyl chloride is reacted with formaldehyde in the presence of zinc chloride, and a process described in Otkrytiya, Izobret., 20, 102 (1987) wherein pivalic acid is reacted with thionyl chloride and paraformaldehyde in the presence of zinc chloride. These processes, however, have had a problem that chloromethyl methyl ether and bis(chloromethyl) ether (these highly toxic ethers are hereinafter referred to as halomethyl ether) are formed as by-products.
Further, in Japanese Patent Application Kokai (Laid-open) No. 152341/1988 is proposed a process wherein an alkali metal salt of an aliphatic carboxylic acid is reacted with a dihalohydrocarbon in a solvent such as alcohol, aliphatic nitrile, ether or the like. It is described in the document that even a dihalomethane can be used as the dihalohydrocarbon. However, our confirmation test for the process revealed that relatively good results were obtained when the dihalohydrocarbon had two or more carbon atoms but, in the reaction with a dihalomethane, only a bis(aliphatic carboxy)methane (a by-product) was formed and substantially no halomethyl ester of an aliphatic carboxylic acid intended by the process was obtained. A similar process is described in Japanese Patent Application Kokai (Laid-open) No. 188044/1991. In this process also, a bis(aliphatic carboxy)methane is formed as a main product.
Furthermore, in Japanese Patent Application Kokai (Laid-open) No. 41052/1991 is proposed a process wherein a metal salt of an alkanoic acid is reacted with a large excess amount of a dihaloalkane in the presence of dimethylformamide or the like at a low temperature for a long period of time (at least 30 hours) to produce a haloalkyl ester of an alkanoic acid. In this process, however, a long reaction time (at least 30 hours) is required; since a dialkylamide of an alkanoic acid such as dimethylformamide or the like is used as a third component, a step for separation and recovery thereof is required; and a yield of an intended product is as low as 16-59%. Thus, the process has not been satisfactory as an industrial process.
The present invention has been made with the aim of providing a process for producing a halomethyl ester of an aliphatic carboxylic acid, which forms no halomethyl ether as by-product (this is highly toxic and highly carcinogenic) and which is advantageous in industrial application.